This invention relates to flow rate control valves which, for example, are often used to control the flow rate of gas in the semiconductor industry, and particularly to a high-temperature flow rate control valve and mass flow controller using a stacked-type displacement device as a drive source and suited to be used at higher temperatures than the normal operating temperature, and to a high-temperature stacked-type displacement device suitable for the use.
A conventional flow rate control valve using a stacked-type displacement device as a drive source is disclosed in, for example, Japanese Patent Laid-open Gazette No. 61-127983.
FIG. 9 is a longitudinal cross-sectional diagram of a main part of an example of the flow rate control valve, or the so-called normal open type of the prior art. Referring to FIG. 9, there is shown a main body 11 which made of, for example, stainless steel and formed in a block shape. This main body 11 has a valve chest 12 which opens upward, an inflow passage 13 communicating with this valve chest 12, and an outflow passage 14. Shown at 15 is a valve seat which is provided under the valve chest 12 and has a valve outlet 16 communicating with the outflow passage 14. Shown at 17 is a diaphragm which is made of a metal and formed to be a thin plate, or like a sheet. This diaphragm is provided to shut tightly the upper part of the valve chest 12 and for its under surface to be fixed to a valve body 18 so that the valve body faces the valve outlet 16. Shown at 19 is a housing which is made of the same material as the main body 11 and formed to be a hollow cylinder. This housing is fixed to the upper side of the main body 11 by a set metal member 20 so as to tightly close the valve chest 12. Shown at 21 is a stacked-type displacement device which has a valve stem 22 fixed to its lower end and which is inserted and set in the housing 19 so that the valve stem 22 can be made in contact with the diaphragm 17. Shown at 23 is an opening adjust screw which is mounted on the upper end of the housing 19 so that its lower end can be made in contact with the stacked-type displacement device 21 by screwing.
With the above structure, a predetermined gap is maintained between the valve seat 15 and the valve body 18, and thus a fluid such as gas is flowed from the inflow passage 13 through the valve chest 12 and valve outlet 16 to the outflow passage 14. Then, when a DC voltage is applied to the stacked-type displacement device 21, the stacked-type displacement device 21 is extended in the stacked-layer direction. Thus, the valve stem 22 is pushed downward to move the valve body 18 downward, so that the gap between the valve body 18 and the valve seat 15, or the opening of the valve outlet 16, is decreased. When the DC voltage is stopped from being applied to the stacked-type displacement device 21, the stacked-type dispalcement device 21 shrinks by the amount corresponding to the expansion by the previous voltage application. Therefore, the valve body 18 returns to the original position by the restoring force of the diaphragm 17, and the opening of the valve outlet 16 is restored to the original state. In this way, the opening of the valve outlet 16 can be adjusted by the DC voltage applied to the stacked-type displacement device 21, so as to control the flow rate of a fluid such as gas from the outflow passage 14.
The stacked-type displacement device used in the flow rate control valve will be described below.
A recently developed stacked-type displacement device, as disclosed in, for example, Japanese Patent publication Gazette No. 59-32040, is produced as follows. A binder is added to raw-material power, mixed and kneaded to produce a paste of a piezoelectric ceramic material, and this paste is formed in a thin plate, or sheet with a predetermined thickness. Then, a conductive material such as silver-palladium is coated on one surface or both surfaces of this plate, or sheet to form the internal electrodes. Several sheets of such thin plates are prepared, stacked, and pressed, and the stacked-layers plate is formed into a certain shape. This stacked, or laminated body is fired to produce an integral ceramic body, and external electrodes are formed on both sides of the laminated body to complete a structure like a laminated chip capacitor. The stacked-type displacement device of this structure is excellent in the adherence between the sheet, or laminar layer of a piezoelectric ceramic material, and the internal electrode, and thus it does not deteriorate over a long period of time.
FIG. 10 shows an example of the structure of a stacked-type displacement device, which is called "the alternate electrode type". In FIG. 10, 31 represents the laminar which is made of a piezoelectric ceramic material. Positive and negative inner electrodes are alternately formed on the laminars, or layers, in turn. These laminars, or layers are stacked to form a laminated body 35. The inner electrodes 32a and 32b are formed to have respective edges to be projected or exposed to the outside on alternate sides of the laminated body and are, respectively, connected to the external electrodes 33a and 33b formed on the sides of the laminated body in the stacked direction. The external electrodes are connected through solder 37 to lead wires 36.
When positive and negative voltages are applied to the external electrodes 33a and 33b, respectively, an electric field is established between the inner electrodes 32a and 32b, so that the sheet, or laminar 31 is expanded, or displaced in the thickness direction by the longitudinal effect of the piezoelectric ceramic material.
FIG. 11 also shows another example of "the stacked-type" displacement device, which is called the all surface electrode type improved in the piezoelectric displacement efficiency (see, for example, Japanese Patent Laid-open Gazette No. 58-196068). In FIG. 11, like elements corresponding to those in FIG. 10 are identified by the same reference numerals. The inner electrodes 32a and 32b are formed on the entire surfaces of the layers, or laminars 31, and a necessary number of sheets of laminars, or layers are stacked, or laminated as above. Then, a coating 34 made of an insulating material is provided on every other edge (for example, only on the edges of inner electrodes 32b) of the inner electrodes 32a, 32b on one side of the laminated body 35 formed as above. In addition, the external electrode 33a made of a conductive material is deposited over the entire surface of the side including the coatings 34. On the other hand, the coating 34 is similarly provided on the every other edge of the remaining inner electrodes (for example, 32a) on the other side of the laminated body 35, and the external electrode 33b is deposited over the entire surface of the side including the coatings 34. The action of this device is the same as that in FIG. 10.
In the conventional flow rate control valve shown in FIG. 9, the valve body 18 reside in the valve chest 12 in which gas is flowed, and this valve body 18 is drawn in or out of the valve outlet 16 which is provided in the valve seat 15, thereby controlling the flow rate. The valve body 18, when cutting off the gas flow, is in contact with the edge of the valve outlet 16, and when controlling the flow rate, it slides against the edge and inner peripheral surface of the valve outlet 16. Thus, friction therebetween causes metal powder due to the abrasion and this metal powder is mixed into the gas. In the normal closed type flow rate control valve, though not shown, the frequency of the contact and slide between the valve body 18 and the valve outlet 16 is considerably large as compared with the normal open type, and therefore a large amount of metal powder is caused by the abrasion, making the above problem even more serious.
Moreover, the amount of displacement of the stacked type displacement device 21 used in the conventional flow rate control valve is at least about 30 to 40 .mu.m even if the length of the laminated body is 40 mm. Since the gas flow rate in the piezoelectric valve of this type is determined by the gap size between the valve seat 15 and the diaphragm 17, only a very small amount of flow can be controlled by the above amount of displacement. Thus, in order to realize a large flow rate controlling pieozoelectric valve, it is necessary to increase the number of layers of the laminars 31 and inner electrodes 32a, 32b which constitute the stacked-type displacement device 21. This inevitably results in a large piezoelectric valve, thus increasing the space to be occupied. Therefore, the conventional piezoelectric valve is used only for controlling a very small flow rate, and it is not suited to use for controlling a large flow rate. For a flow rate, or for controlling a wide range of flow rate, several piezoelectric valves are necessary, thus not only occupying a large space or floor area but also making the maintenance and inspection and management complicated.
In recent years, in the semiconductor manufacturing field, purer and higher-temperature reaction gas is required for use, and the instrument or apparatus for supplying the gas is desired to withstand high temepratures. When the conventional flow rate control valve is used in a high temperature range of, for example, 100.degree. C. or above, the amount of displacement and static capacitance of the electromechanical transducing material of which the laminar 31 is made is considerably changed by temperature, so as not to properly function. In other words, because the Curie temperature (the temperature at which the piezoelectric characteristic is lost) of, for example, the common electromechanical transducing material is about 150.degree. C., the piezoelectric distortion constant, d33 is suddenly decreased when the temperature of the device exceeds 100.degree. C., and thus the amount of displacement is greatly decreased to far less than a necessary amount. Particularly in recent years, the use of the flow rate control valve is expanded up to the high temperature range of about 200.degree. C., and the specification thereof becomes more servere than before. Thus, it is highly desireable to realize a flow rate control valve having a stable control function against any ambient temperature at which the device is used.
In addition, the solder 37 connecting the external electrodes 33a, 33b and the lead wire 36 can melt at the high temperature, disconnecting the electrodes and the lead wire. Moreover, in devices covered with a film of an epoxy resin (epoxy aromatic diamine, polyamine, nyron or aliphatic amine family) for increasing the strength against, for example, a high humidity atmosphere, the film can melt or peel off so that the device is easily deteriorated.